A target of 50,000 new homes each year can solve the capital's housing crisis Jeremy Selwyn

The next Mayor of London must “think the unthinkable” and encourage building on green belt land to have any chance of hitting the 50,000 new homes a year target needed to solve the housing crisis, a major new report says today.

It warns that redundant “brownfield” sites cannot possibly deliver the house building explosion that all the main mayoral candidates have promised to deliver if elected on May 5.

So far Labour’s Sadiq Khan, Tory Zac Goldsmith, Lib-Dem Caroline Pidgeon as well as the Green Party’s Sian Berry have explicitly ruled out allowing any building on undeveloped green land as part of their plans to fix the crisis.

But analysis in the report, commissioned by the housing charity Shelter, concludes that a “not credible” equivalent of four brownfield mega-developments a year on the scale of the Olympic Park in Stratford would be needed to reach the target.

Other politically unpopular options such as hundreds more tall buildings, a new generation of “garden towns” and a huge increase in the density of building in the suburbs will also have to be adopted by the next mayor if their election pledges are to have any credibility, the analysis warns.

But it will be the call from the respected charity to build on some of the capital’s 70,000 acres of green belt land — more than a fifth of London’s total area — that will prove most incendiary.

Shelter’s chief executive Campbell Robb said: “Decades of failure to build enough homes means that there’s no silver-bullet solution when it comes to fixing London’s chronic housing shortage. If a candidate’s setting out a solution that seems easy, they’re not doing enough.”

Analysis in the Shelter report from London planning expert Barney Stringer of the Quod consultancy suggests that suitable brownfield sites are rapidly running out because many, such as those at King’s Cross, Stratford and Nine Elms, have already been earmarked for housing.

Only about four to five per cent of London’s developed brownfield land could be used for new homes.

To date only the Right-wing think tank the Adam Smith Institute has advocated the alternative of sacrificing some of London’s heavily-protected “green lung” for housing.

But today’s Shelter report said a smaller scale repeat of the “Metroland” expansion of London in the Thirties could make a huge contribution.

It said: “Before the metropolitan Green Belt was established London saw unprecedented rates of development.

"Almost one in five of London’s current homes were built in a single 10-year period before the Second World War. A much smaller and more controlled release of Green Belt could be an effective way to deliver substantial numbers of new homes.”

The report adds: “There is a legitimate debate about whether London’s Green Belt could be better managed, ensuring the protection of beauty and public access as well as providing new homes. The new mayor will need to take a pragmatic rather than absolutist view.”

At a housing debate last week all the major candidates promised to double house building from the current level of around 25,000 to 50,000.